Extract manifest path resolution logic from process test run() function

[coderabbitai]

The manifest path resolution and creation logic in the run() function (lines 58-74 in tests/steps/process_steps.rs) should be extracted into a dedicated test helper to reduce verbosity and centralize setup logic.

Currently, the run() function contains approximately 15+ lines of inline logic for:

• Computing absolute manifest paths
• Creating missing manifest files
• Persisting temporary files

This logic should be moved to a reusable test helper function to improve maintainability and reduce code duplication.

Backlinks:

• Original PR: Handle manifest path in process tests #56
• Discussion: Handle manifest path in process tests #56 (comment)
• Issue: Extract manifest path resolution logic from process test run() function #60

Requested by: @leynos